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Part L is being revised. Some might say it's about time, as this is the only important piece of legislation on energy use in new buildings. And some have already said it's too little, too late, because the carbon savings target it is intended to contribute to 鈥� up to 1.32 million tonnes of carbon per year 鈥� is still not high enough to prevent climate change (se factfile).

Since the Regulations affect all types of construction, it is important that changes are introduced only after due consultation. This the government did between June and September last year by inviting comment from interested parties 鈥� everyone from the Construction Industry Council to the concerned self-builder.

The intention is not to obstruct the government's proposed tightening of the rules, it is more to give the industry a chance to comment on the detail and timetable for its introduction. This is proving to be much harder than anticipated.

The DETR has confirmed that the 1 April date for publication of the new legislation will be missed. But environment minister Michael Meacher has announced in the House of Commons that "we hope to make first amendments [to Part L] in the summer". So, officials are rushing to evaluate the feedback and decide on implementation 鈥� an assignment complicated by the simultaneous consultation on the acoustic regulations (Part E, pages 16-19) 鈥� which allows further comment on the Part L.

As time ticks on, debate is raging over whether the energy regulations should be split into separate documents covering dwellings and non-dwellings. It is calculated that dwellings would account for 58% of the carbon savings mooted, making the domestic market the main target for achieving change.

There is also a debate about the timing of the changes. The original plan was for two phases: implementation six months after the rules are published, and a tightening 18 months after publication. But industry pressure has forced the government to abandon the second phase. The tighter values that would have been introduced later will now come into force as the first set of changes 鈥� except for walls, which will be introduced at a lower insulation value than was originally proposed.

So what practical effect will the changes have on designers trying to achieve sustainable architecture? The widening of the "material alteration" definition will mean, for example, that replacement windows will be required to conform to the new U-values (1.8-2.0 W/m²掳C). This is intended to encourage the use of low-emissivity rather than plain glass, although it is acknowledged that these values may not be appropriate in certain conservation instances.

Also, there are specific rules controlling the addition of conservatories. The conservatory either has to perform as well as the external fabric or must be separated from the house by well-detailed double-glazed doors, for example. Any extension and refurbishment that affects the building fabric, both domestic and commercial, will need to comply with the more onerous requirements for insulation.

Higher U-values can only be a good thing. The impact on construction is likely to be seen in several ways. First, building envelopes will become thicker as insulation thicknesses are increased to achieve the new values. Tables in the consultation document give required thicknesses for materials and types of construction, although the range is limited.

Timber frames can be more easily adapted to meet the new insulation values, leading to concerns that masonry construction will prove uneconomic (see Panic over). Manufacturers of blocks, insulation and cladding materials have already been asked for better-performing products, so we can expect to see new products launched. For windows and rooflights, where a U-value of 2.0 W/m²掳C is expected, triple glazing will become more feasible on thermal as well as noise-reduction grounds, with specialist low-emissivity glass promoted as the most appropriate to meet the new standards.

The proposal to improve air-tightness is hitting the Achilles heel of many a well-intentioned low-energy design. Without restricting uncontrolled ventilation losses through the fabric, all the effort that goes into energy conservation measures is 鈥� literally 鈥� so much hot air. Designers will be required to demonstrate that a building meets the air leakage index target of 10 m³/h per m² of the building fabric at an applied pressure of 50 Pa. Testing for airtightness is a straightforward exercise using a mobile testing unit. Under the new legislation, building control bodies will have powers to carry out testing, and to require remedial measures if tests are not passed 鈥� with a transition period for "improvement" allowed.

The measures also refer to the importance of site workmanship and avoiding breaches in the insulation by encouraging good practice in design detailing 鈥� a government-backed publication of "robust standard details" is imminent 鈥� and assisting in raising standards of cladding. Compliance may be achieved through testing, or through self-certification, whereby a "competent person" confirms that appropriate fixing techniques and design details have been used. However, at this stage, it is difficult to see how improved site workmanship can be enforced without enormous resources being given to building control bodies.

For non-domestic buildings, the revisions discourage reliance on mechanical cooling systems in guidelines on how summertime over-heating can be avoided through the use of shading, orientation, thermal mass, night cooling and the like. This is the first time that the Regulations have directly addressed energy use for cooling, and for the sustainability-minded designer, it is a golden opportunity to promote passive systems to more clients.

New standards for minimum boiler efficiency will require designers to select an inherently efficient unit and fuel source, or carry out lengthy calculations to prove the compliance of less energy-efficient units. For example, a coal-fired boiler will not meet the required standards (expressed as a maximum carbon intensity 鈥� the carbon emitted per useful kWh of heat output). The proposals do allow some trade-off between system efficiencies and fabric performance 鈥� a more efficient boiler selection can lead to a slight relaxation of the minimum U-values 鈥� but this proposal has not been welcomed by conscientious designers, who point out that heating systems are often replaced, whereas building fabric is not.

The standards for light fittings will require selection of at least some low-energy fittings 鈥� fancy tungsten halogens are not eliminated but their use is to be limited 鈥� with a calculation required to show lighting averages of less than 40 luminaire-lumens per circuit-watt. This should lead to more interesting designs for efficient light sources as manufacturers seek to promote their luminaires as truly green.

The new carbon performance index method of measuring air-conditioned or mechanically ventilated buildings brings a new raft of calculations to the building services engineer's fingertips. No doubt these will shortly be available as add-ons to software packages. The implication for designers, however, is not so predictable. Theoretically, it should give the engineer more freedom to provide alternative means of complying with Part L, and enable unusual buildings to be assessed as a whole, taking into account useful heat gains and overall performance. In practice, it will take some time for engineers to become familiar with the new methods and longer still to develop a feel for what the figures mean. However, linking the efficiency of mechanical systems to carbon emissions should give a more transparent comparison of the various options.

Proposals for commissioning should reinforce existing good practice, although the demands on self-certification may prove difficult for installers. For one thing, the "competent person" who submits reports confirming effective commissioning will need training.

The new requirements for energy meters and building log-books will no doubt lead to further scrambles towards the end of projects as contractors struggle to collate as-built information and operation and maintenance manuals, yet their introduction is crucial if building owners and operators are to monitor energy consumption against a benchmark figure and thus be alerted to poor performance in good time. The exact benchmark to be set for comparison is a matter of some debate and mandatory reporting is not planned until a later date, but the introduction of these devices now will make crucial feedback available sooner rather than later.

For dwellings, the major impacts will be seen in the improved U-values, although here too there is some scope for trade-off with boiler efficiencies. For the first time, guidance on energy-efficient lighting in homes has been included, in the form of a table showing the minimum number of fittings to be provided that only accept energy-efficient bulbs. We can expect to see improved decorative quality for compact fluorescent luminaires as a result. More specific regulations covering heating and hot water system controls are to be introduced, with separate timers required for different uses. So, for example, sleeping and living areas should have independent time controls, and boilers should operate only where there is demand for heat, allowing a more efficient regime.

Domestic installers will be required to self-certify that heating and hot water systems meet the relevant standards. This will introduce new paperwork that may be too onerous, as it is not yet clear how this will affect the do-it-yourself market. However, the insistence on the provision of clear information on efficient ways to operate installed heating systems should help individuals to control their energy bills better.

The government has calculated that the impact on cost could be up to 拢1400 per dwelling, or 拢7/m² for non-domestic buildings. This may cause a sharp intake of breath, but when compared to annual running-cost savings of 25% 鈥� not including the impact of the climate change levy 鈥� it is clear that the benefits quickly outweigh the costs of the changes. And if that isn't sufficient incentive, the benefits to the planet should be.